This invention relates to an improved apparatus for producing optical fibers using a multiple crucible.
The crucible method for production of an optical fiber wherein a glass material is melted by heating in a crucible and the molten glass is permitted to flow down from a spinnerette provided through the bottom wall thereof, while being spun into an optical fiber, is susceptible to the following three main factors for operation in producing a high quality fiber with uniform cross-sectional area along the entire length. (1) The viscosity of the molten glass is stabilized with time. (2) The operation of spinning a glass fiber from the spinnerette of the crucible is stabilized with time. (3) The level of the molten glass in the crucible is maintained constant during the entire spinning operation. Of these, the first factor depends upon temperature, and therefore a sufficient viscosity stabilization will be effected by the temperature control with improved accuracy. The second factor, i.e., the drawing operation can be performed with considerable stability by using an automatic spinning mechanism of the drum type. Unlike these two factors, the third factor, i.e., the maintenance of a constant level of the molten glass has theretofore been difficult to perform in practice, and therefore desired results can not be effected. This difficulty becomes serious in producing uniform fibers along the length thereof.
To improve the uniformity of optical fibers, it has been the practice of the prior art either to control the rate of material supplied continuously or intermittently in accordance with the amount of spun fiber as determined by the operator's judgement, or to control the amount of material supplied while observing the level of the molten material. The former supply control is associated with a difficulty of balancing the used-up and supply amounts with each other. In the latter connection, the molten glass as well as whole the crucible is incandescently so luminous that it is difficult to determine where the level of the molten glass is with sufficient accuracy. Thus these prior art techniques are incapable of maintaining the level of molten glass constant with sufficient accuracy during the entire spinning operation.